1. Field of the Invention
The invention concerns a UV-reflective interference layer system for transparent substrates with broadband antireflection in the visible wavelength range, a method for coating a substrate with such a layer system, and the use of such coating systems in various fields of application.
2. Description of the Prior Art
Currently known glass antireflections for the visible spectral range, such as the MIROGARD or the AMIRAN antireflection of Schott-DESAG AG, Grünenplan, are interference filters of three layers, wherein a layer with an intermediate index of refraction is first deposited, followed by a layer with high index of refraction, usually TiO2, and then a layer with low index of refraction, usually SiO2 or MgF2. As the layer with intermediate index of refraction, for example, a mixture of SiO2 and TiO2, but also Al2O3 is used. Such three-layer antireflections are deposited, for example, on eyeglass lenses, on monitors, on plate glass, such as display window panels, on treated lenses, etc.
In most instances, these filters have a blue-violet or green residual reflection. When light impinges perpendicularly, the reflection characteristic of glasses coated on both sides is characterized in that the reflection within the wavelength interval of around 400–700 nm is less than 1%, for example, but outside this range the reflection rises to values of up to around 30% (V or W-shaped characteristic), i.e., far above the 8% of uncoated glass.
The drawback to such systems is that, when viewing at an angle that increasingly deviates from the perpendicular, the characteristic shifts to ever shorter wavelengths, so that the long-wave reflection maximum ends up in the visible range, and produces an undesirable red component to the reflected light color.
One goal of the present invention is therefore to find an antireflection whose residual reflection is low in a much broader wavelength range, i.e., in the range from 400 to at least 800 nm with perpendicular incidence of light, and which furthermore also provides broadband antireflection at rather large viewing angles. In many applications, such as display window glazings or glazings for pictures, a neutral-color appearance is in fact desirable, especially for different viewing angles.
Especially for picture glazings, say, in museums, but also in the case of display window glazings, furthermore, it is desirable that an antireflecting glass—if possible, color-neutral—at the same time provides the function of protecting the colors of the picture or the natural or synthetic fibers, as well as the dyestuffs of the window displays, against ultraviolet light.
As is known, the UV component of sunlight or that of lamp light, especially in the case of metal halide or other gas discharge lamps, but also even with halogen bulbs, is sufficient to cause considerable damage over a lengthy period of time, such as discoloration or embrittlement of natural or synthetic fabrics. A UV protection would also be desirable for glazings in office or residential buildings, in order to greatly reduce the fading of wood surfaces, draperies, upholstered furniture, etc., under direct sunlight, and thus enable, for example, an improved passive utilization of solar energy. Present-day thermal protection glasses, which contain a thin silver layer, are not antireflective in the visible range, and furthermore also do not offer sufficient UV protection, since thin silver layers become transparent in the UV.
In the case of known antireflective soft glass, UV protection is achieved by the use of organic polymers as absorbers of UV light, for example, as compound glass, wherein two glass panes are laminated together with a PVB plastic foil adapted by its index of refraction to the glass, for example, 380 μm in thickness (the glass MIROGARD-PROTECT from Schott-DESAG). Such glasses are [used] under intense lamp light, for example, as front panels for lamps, but they are not temperature-stable and they are also degraded by intensive UV radiation. Also, their three-layer antireflection on one side has the above-mentioned limitations, and furthermore the production of compound glass is costly.
Another possibility is the use of UV-absorbing varnish layers, which are several micrometers thick and are transparent to visible light. Such varnish layers are likewise not stable to UV and temperature, and after being deposited on the glass they must further be made antireflective. Regarding the state of the art, refer also to the following publications:                D1: H. Schröder, “Oxide Layers Deposited from Organic Solutions”, in Physics of Thin Films, Academic Press, New York, London, Vol. 5 (1969), pp. 87–140        D2: H. Schröder, Optica Acta 9, 249 (1962)        D3: W. Geffeken, Glastech. Ber. 24, p. 143 (1951)        D4: H. Dislich, E. Hussmann, Thin Solid films 77 (1981), pp. 129–139        D5: N. Arfsten, B. Kaufmann, H. Dislich, Patent DE 3300589 C2        D6: N. Arfsten, B. Lintner, et al., Patent DE 4326947 C1        D7: A. Pein, European Patent 0 438 646 B1        D8: I. Brock, G. Frank, B. Vitt, European Patent 0 300 579 A2        D9: Kienel/Frey (ed.), “Dünnschicht-Technologie [Thin layer technology]”, VDI-Verlag, Düsseldorf (1987)        D10: R. A. Häfer, “Oberflächen- und Dünnschicht-Technologie [Surface and thin layer technology]”, Part I, “Coating of Surfaces”, Springer-Verlag (1987)whose disclosure contents are fully incorporated in the present application.        